In World War II the UK government built and sank a number of (supposedly temporary) sea forts in the Thames Estuary. One of the forts (HM Tongue Sands) was visible several miles off the shore in my home town of Margate.
In 2007, one of the forts, Sealand was up for sale...the founder, who is old and lives in Spain, having decided to get out of the country management business.
I wrote a detailed document about North Sea forts, using information freely available on the Internet, and gave it to my dad as a Christmas present 3 years ago. I may publish it to this blog if there is interest in making it available.
In the meantime,here is a collection of photos from some of the sea forts.